Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with nonstick cooking spray.
In a medium bowl, combine the hot milk and old-fashioned oats. Let the mixture soak while you prepare the remaining ingredients.
In another medium bowl, whisk together the flour, cinnamon, baking soda, and salt.
In a large bowl, use an electric mixer to cream together the butter, brown sugar, and granulated sugar. Add the eggs one at a time, beating well after each addition. Mix in the vanilla extract.
Gradually add half of the flour mixture to the creamed butter mixture, followed by the soaked oats (including any remaining liquid), and then the remaining flour mixture. Mix until just combined.
Pour the batter into the prepared baking dish and bake for 35-40 minutes, or until the cake is set and a toothpick inserted into the center comes out clean.
While the cake is baking, prepare the streusel topping. In a saucepan, melt the butter over medium heat. Add the brown sugar and milk, stirring until the sugar is fully dissolved. Remove from heat and stir in the coconut flakes and chopped pecans.
Once the cake is done baking, remove it from the oven and immediately spread the streusel topping evenly over the cake. Let the cake stand for about 10 minutes before slicing and serving.